1. Field of the Invention
This invention relates to a semiconductor device, specifically to a semiconductor device provided with a multi-layer wiring structure in which a plurality of wiring layers is stacked through interlayer insulation films.
2. Description of the Related Art
An IC (Integrated Circuit) chip is provided with bonding pads (external terminals) which serve to supply a multitude of semiconductor elements formed in the IC chip with predetermined voltages (a power supply voltage, a ground voltage and the like) and various electric signals or to output various electric signals to outside of the IC.
FIG. 4 is a cross-sectional view showing an outline of a region of a conventional semiconductor device, in which a bonding pad is formed. Device elements 101 composed of a multitude of semiconductor elements such as transistors and capacitors are formed on a surface of a semiconductor substrate 100. A first wiring layer 102 that is electrically connected with the device elements 101 through a wiring (not shown) is formed on the surface of the semiconductor substrate 100 through an insulation film 103 made of a silicon oxide film or the like.
A first interlayer insulation film 105 having via holes 104 is formed on the insulation film 103 to cover the first wiring layer 102. A conductive layer 106 made of tungsten or the like is formed in each of the via holes 104. A second wiring layer 107 electrically connected with the first wiring layer 102 through the conductive layer 106 is formed on the first interlayer insulation film 105. Similarly, a second interlayer insulation film 108 is formed on the first interlayer insulation film 105, a third wiring layer 109 and a third interlayer insulation film 110 are formed on the second interlayer insulation film 108 and a fourth wiring layer 111 is formed on the third interlayer insulation film 110. The fourth wiring layer 111 serves as the bonding pad. The fourth wiring layer 111 is disposed in an area overlapping the device elements 101 for a purpose of reducing a die size. A protection film 113 having an opening on the fourth wiring layer 111 is formed on the third interlayer insulation film 110.
A test probe or a lead wire touches the fourth wiring layer 111 exposed in the opening 112 during probe testing or wire bonding. During the probe testing, a large mechanical stress is caused by a pressure of the test probe made of tungsten, nickel alloy or the like and converges in a region below the fourth wiring layer 111. Also, during the wire bonding, a large mechanical stress converges in the region below the fourth wiring layer 111 by influence of ultrasonic energy or the lead wire. As a result, in some cases, cracks 114 are caused in the third interlayer insulation film 110 under the fourth wiring layer 111.
Furthermore, in some cases, the cracks 114 reach the interlayer insulation film (the second interlayer insulation film 108 or the first interlayer insulation film 105) or the wiring layer (the third wiring layer 109, the second wiring layer 107 or the first wiring layer 102), that is further below the third interlayer insulation film 110. In those cases, the cracks 114 cause deterioration in metal migration resistance or a short circuit failure between wirings, because a corrosive material such as water infiltrates into the inside through the cracks 114.
Following technologies are proposed to suppress the deterioration in reliability due to the cracks 114. One of the technologies is implemented by a structure in which approximately the same size of the third wiring layer 115 as the fourth wiring layer 111 is disposed so as to overlap each other and the conductive layers 116 that electrically connect the fourth wiring layer 111 with the third wiring layer 115 are disposed only below the protection film 113 in a shape of rectangular rings as shown in FIG. 5 and FIG. 6. Even if the cracks 114 are caused, with the structure provided with the third wiring layer 115 and the conductive layer 116 in the shape of rectangular rings, the downward propagation of the cracks 114 would be blocked by the third wiring layer 115. FIG. 5 is a cross-sectional view of a section Y-Y shown in FIG. 6. Only the fourth wiring layer 111, the conductive layer 116 and the protection film 113 are shown in FIG. 6 for the sake of simplicity.
Also, there is proposed a structure in which a multitude of rectangular rings of conductive layers 117 that electrically connect the fourth wiring layer 111 with the third wiring layer 115 is disposed over approximately all the area that overlaps the fourth wiring layer 111, as shown in FIG. 7 and FIG. 8. With the structure provided with the multitude of rectangular ring-shaped conductive layers 117, the downward propagation of cracks 114 is blocked by the third wiring layer 115 while lateral propagation, that is the propagation in directions parallel to the surface of the semiconductor substrate 100, of the cracks 114 caused in the third interlayer insulation film 110 is suppressed by the rectangular ring-shaped conductive layers 117. FIG. 7 is a cross-sectional view of a section Z-Z shown in FIG. 8. Only the fourth wiring layer 111 and the conductive layers 117 are shown in FIG. 8 for the sake of simplicity
The technology mentioned above is described in Japanese Patent Application Publication No. H06-196525, for example.
Probe testing showed that the mechanical stress imposed on the bonding pad was not sufficiently relaxed by the third wiring layer 115 with the conventional structures described above.
In recent years, in order to reduce a die size, bonding pads have come to be formed over a region where the device elements are formed. Therefore, the mechanical stress reaches the device elements if the third wiring layer 115 can not sufficiently relax the mechanical stress imposed on the bonding pad. Dimensions of semiconductor elements constituting the device elements and connecting wirings have been reduced year by year. They are susceptible to failure and their electric characteristics (a threshold voltage of a transistor, for example) are prone to be varied when the mechanical stress reaches them.